mm/hugetlb: fix hugetlb_pmd_shared()

Patch series "mm/hugetlb: fixes for PMD table sharing (incl.  using
mmu_gather)".

One functional fix, one performance regression fix, and two related
comment fixes.

I cleaned up my prototype I recently shared [1] for the performance fix,
deferring most of the cleanups I had in the prototype to a later point. 
While doing that I identified the other things.

The goal of this patch set is to be backported to stable trees "fairly"
easily.  At least patch linuxppc#1 and linuxppc#4.

Patch linuxppc#1 fixes hugetlb_pmd_shared() not detecting any sharing
Patch linuxppc#2 + linuxppc#3 are simple comment fixes that patch linuxppc#4 interacts with.
Patch linuxppc#4 is a fix for the reported performance regression due to excessive
IPI broadcasts during fork()+exit().

The last patch is all about TLB flushes, IPIs and mmu_gather.  Read:
complicated

I added as much comments + description that I possibly could, and I am
hoping for review from Jann.

There are plenty of cleanups in the future to be had + one reasonable
optimization on x86.  But that's all out of scope for this series.


This patch (of 4):

We switched from (wrongly) using the page count to an independent shared
count.  Now, shared page tables have a refcount of 1 (excluding
speculative references) and instead use ptdesc->pt_share_count to identify
sharing.

We didn't convert hugetlb_pmd_shared(), so right now, we would never
detect a shared PMD table as such, because sharing/unsharing no longer
touches the refcount of a PMD table.

Page migration, like mbind() or migrate_pages() would allow for migrating
folios mapped into such shared PMD tables, even though the folios are not
exclusive.  In smaps we would account them as "private" although they are
"shared", and we would be wrongly setting the PM_MMAP_EXCLUSIVE in the
pagemap interface.

Fix it by properly using ptdesc_pmd_is_shared() in hugetlb_pmd_shared().

Link: https://lkml.kernel.org/r/20251205213558.2980480-1-david@kernel.org
Link: https://lkml.kernel.org/r/20251205213558.2980480-2-david@kernel.org
Link: https://lore.kernel.org/all/8cab934d-4a56-44aa-b641-bfd7e23bd673@kernel.org/ [1]
Fixes: 59d9094 ("mm: hugetlb: independent PMD page table shared count")
Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
Tested-by: Laurence Oberman <loberman@redhat.com>
Reviewed-by: Rik van Riel <riel@surriel.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prakash Sangappa <prakash.sangappa@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Cc: Uschakow, Stanislav" <suschako@amazon.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: David Hildenbrand (Red Hat)

include/linux/hugetlb.h

@@ -1326,7 +1326,7 @@ static inline __init void hugetlb_cma_reserve(int order)
 #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
 static inline bool hugetlb_pmd_shared(pte_t *pte)
 {
-	return page_count(virt_to_page(pte)) > 1;
+	return ptdesc_pmd_is_shared(virt_to_ptdesc(pte));
 }
 #else
 static inline bool hugetlb_pmd_shared(pte_t *pte)